To date, communication and computer systems have played a dominant role in many fields. At the same time, among various microstructure optoelectronic technologies, integrated optics represents a promising approach in these advanced information processing areas. In these systems, Si-based logic an memory integrated circuitry continues its evolution toward higher speed and enhanced functionality, with a resulting decrease in feature size and increase in process complexity. Future electronic systems will require on-chip signal conversion between electrical, optical and microwave media to reach the speed and functionality projections. Thus a radically different alternative concept exploits the use of photons, instead of electrons, to carry information in what is commonly referred to as "optical interconnects." One implementation of this strategy relies on the integration of crystal-, semiconductor- or polymer-based optoelectronic interconnects on a host Si substrate, and thus requires feasible crystal-, semiconductor- or polymer-based optoelectronic technologies in order to produce Si-based photonic modulators for optical waveguide interconnects.
Although the technologies for some electro-optic (EO) waveguide devices based on inorganic materials such as crystals and semiconductors have had a long developing history, the conditions for manufacturing and processing integrated optical devices are also seriously limited. While polymers, a new kind or organic nonlinear EO materials, not only have high EO nonlinearity, but also high thermo-optic (TO) effect, and have shown a promising future. Polymers generally have potentially both large EO and TO coefficients, low dielectric constants, improved thermal and temporal stability, and easy fabrication conditions. The above physical properties of polymers are very useful in constructing waveguide-type optical functional devices such as modulators and switches. A variety of polymer-based modulators aimed at providing feasible structures with high-extinction ratios have been reported. The technologies associated with packaging and interfacing with other devices are also taken as important considerations. These EO modulators focus mainly on two types: phase modulators and intensity-modulators.
For many applications, the required distance and bandwidth are within the operating parameters of both single-mode and multi-mode optical systems. High extinction-ration modulators are always needed in both single- and multi-mode fiber-optic communication systems. Therefore, the structures that can be suitable for both single-mode and multi-mode waveguide modulators will have wide applications in industry. As various fiber-optic communication systems are developed and applied in the real world, the interesting needs of high capacity in these systems require both more information channels in a single fiber and higher bit rate in each channel.
In telecommunications networks, the time-division-multiplexing (TDM) systems have been successfully used according to the SONET-standards. Among the high transmission rates for TDM systems, 2.5 Gbits/s is relatively popular according to OC-48-standard and the new transmission systems having up to 10 Gbits/s are widely applied according to OC-192-standard. The wavelength-division-multiplexing (WDM) lightwave system is the optical communication in the wavelength multiplex mode. Use of this novel approach WDM has the potential of improving the performance of the fourth generation lightwave systems by a factor of more than 1000. Recently, research on the devices and techniques for high capacity WDM systems or dense wavelength division multiplexing (DWDM) systems having effective network restoration capability, i.e., reconfigurable WDM systems, has received much more attention. In future, the hybrid fiber-optic communication systems including both ultra-high bit-rate TDM and high capacity WDM (or DWDM) systems, the routing of optical signals will be performed in optical cross-connects (OXCs). The ultra-high speed operations of the TDM systems will open a huge market for a variety of high performance EO waveguide modulators. The functions and applicability of the WDM systems will be extended by the reconfigurable structures. Therefore, the single high-performance EO waveguide switching devices and the programmable OXCs using the EO switching cells will have wide applications in fiber-optic communication.
In accordance with theoretical study, the coupling efficiency between two single-mode waveguides can be achieved a high value only at the critical coupling length, while the total coupling efficiency between two multi-mode waveguides cannot achieve a high value at all.
A paper titled "Polymeric optical intensity modulator optimized in quasi-single mode operation" by W. Hwang et al., published in Appl. Phys. Lett. 69 (11) (1996), pp. 1520-1522, discusses an EO polymer waveguide intensity modulator and is incorporated herein by reference.